1,721,186 research outputs found
Geometric structure and optical properties of the GaAs(001)-c(4 X 4) surface
No full textWe consider the electronic structure and optical properties for several geometries of the GaAs(001)-c(4x4) surface using first-principles calculations. We find strong evidence that the best agreement with photoemission and reflectance anisotropy spectroscopy experiments carried out on surfaces prepared under As-4 flux is obtained for a structure containing three Ga-As dimers per unit cell. The standard As-As dimer model yields similar, but distinguishable results, while an asymmetric dimer model is found to yield completely incompatible surface spectra
The transition from 3C SiC(111) to graphene captured by ultra high vacuum scanning tunneling microscopy
No full textIn this paper we clarify the transformation mechanism of 3C-SiC into graphene upon thermal decomposition, by a combination of high resolution Scanning Tunneling Microscopy (STM) images and first principle calculations. We studied the transition from 3C-SiC to graphene by high temperature annealing of C-terminated 3C SiC (1 1 1)/Si (1 1 1) samples in Ultra High Vacuum. By using STM we were able to observe very clear atomic resolution images of the transition from SiC (v33)R30࠴o a new intermediate stage SiC View the MathML source (very close to the graphene (2 נ2) reconstruction) after annealing at 1250 î We also obtained images of the transformation of the intermediate structure into a (1 נ1) monolayer graphene, caused by further sublimation of atoms in the subsurface layer. We have interpreted the results by using Density Functional Theory - Local Density Approximation calculations, which give full account of the SiC (v33)R30࠲econstruction, but fail to describe the SiC View the MathML source structure due to its incommensurability with the 3C-SiC (1 1 1) lattice.Full Tex
Annealing effects on faceting of InAsGaAs (001) quantum dots
No full textThe aspect ratio and faceting evolution of quantum dots grown at 500 degrees C were studied as a function of postgrowth annealing temperature. We show that faceting and aspect ratio strictly depend on growth conditions. The evolution toward {136} and {137} facets is kinetically limited and occurs under different experimental conditions. Furthermore long annealing procedures lead to the occurrence of low aspect ratio domes different from those forming at higher growth temperatures
In-line correlation and ordering of InAs/GaAs multistacked Quantum Dots structures
No full textSeveral multilayer InAs/GaAs(001) samples were grown in a Molecular Beam Epitaxy chamber under critical growth conditions in order to induce the self-assembling of chains of InAs Quantum Dots over mounded GaAs surfaces. As the number of deposited layers was increased, an increasing in-line ordering was observed. Finite Element Method simulations confirmed this trend which is driven mainly by the propagation of the elastic strain field through the layers. On the other hand, the morphological features of the surface contribute to improving the alignment of the InAs Quantum Dots in every chain
Scaling behavior of GaAs and GaMnAs quantum rings grown by droplet epitaxy
No full textThe transition from the liquid phase of Ga droplets to the formation of GaAs and GaMnAs
quantum rings has been studied as a function of temperature. We show that different aggregation
processes involve the GaAs (GaMnAs) island and the droplet formation. Furthermore, the aspect
ratio of the islands exhibits an anomalous scaling law related to a tendency to aggregate in the
vertical direction
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Electronic anisotropy of the GaAs(001) surface studied by energy loss spectroscopy
No full textHigh-Resolution Electron-Energy-Loss Spectroscopy (HREELS) has been applied to investigate the anisotropy of the GaAs(001)-c(4 x 4) and beta2(2 x 4) reconstructions. Measurements have been performed on high-quality samples grown in situ by Molecular Beam Epitaxy. The loss intensity is different in the directions parallel and perpendicular to dimers, particularly close to the fundamental gap. We construct relative difference intensity spectra which can be directly compared with the differential reflectivity spectra of the RAS spectroscopy. A one-to-one correspondence is found between experimental and calculated electronic transitions up to about 3 eV. The surface anisotropy given by EELS is about two orders of magnitude higher than that measured optically. The contributions to the anisotropy originate entirely from a few atomic layers beneath the surface. In the beta2 phase we find direct evidence of transitions involving the dimers of the top atomic layer which are well separated by those involving bulk states modified by the surface. (C) 2003 Elsevier Science Ltd. All rights reserved
Apparent critical thickness versus temperature for InAs quantum dot growth on GaAs(001)
No full textWe studied the temperature dependence of the two-dimensional to three-dimensional growth transition in InAs/GaAs(001) heteroepitaxy by means of reflection high energy electron diffraction and atomic force microscopy. The observed shift of the transition to higher InAs deposition times, at temperatures above 500 degrees C, is not a change of critical thickness for islanding, which instead, is constant in the 450-560 degrees C range. Consequently, In-Ga intermixing and surface and interface strain have a negligible dependence on temperature in this range
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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